The output beam of an edge-emitting diode-laser is typically delivered from an emitting area (output-face) thereof which is much wider than it is high. The output beam is highly asymmetric, diverging much less in the width direction of the output-face than in the height direction of the output-face. The width and height directions are commonly referred to by practitioners of the art as the slow and fast axes respectively of the diode-laser.
In many so-called diode-laser packages or modules, the asymmetric output beam of one or more diode-lasers is coupled into one or more optical fibers. Transmission of the beam along an optical fiber, even for a relatively short length, "homogenizes" the beam, such that when it is delivered from the end of the fiber the beam has a relatively symmetrical distribution. The output of the optical fiber (or in the case of multiple diodes-per-module a fiber array or bundle) provides the output aperture of the module. For efficient coupling, it is required that the beam lateral dimension and beam divergence in both the fast and slow axes be equal to or less than the fiber diameter and numerical aperture where the beam is launched into the fiber.
In many arrangements, a diode-laser beam is coupled longitudinally into the end of a fiber. In so-called "butt coupling" arrangements, the fiber end-face is simply placed as close to the emitting-area of the diode-laser as possible, and the fiber is sized to collect as much of the beam as possible. This arrangement is not particularly efficient.
In a more efficient longitudinal-coupling arrangement, a cylindrical microlens, which may itself be a short length of optical fiber, is aligned close to the diode-laser emitting-area parallel to the width direction thereof. The end of the optical fiber into which the radiation is to be coupled is placed close to the microlens. The microlens has optical power only in the fast axis of the diode-laser and is arranged to collect as much as possible of the output beam in that axis and collimate or focus the output beam into the optical fiber. As the microlens has no optical power in the slow axis of the diode laser, the optical fiber must be sized, and have a numerical aperture sufficient, to collect as much as possible of the beam in the slow axis. This coupling method is commonly used for coupling output beams from a longitudinal array of diode-lasers (a so-called diode-laser bar) into an array of optical fibers. A single length of optical fiber can serve as the cylindrical microlens lens for all diode-lasers in the array.
A problem with the single microlens is that it can be difficult to get all of the desired focusing power into a single microlens. The lens can be too small to be conveniently handled, and may need to be positioned too close to the laser diode for convenience. Another problem with a single microlens is that the beam divergence and beam lateral dimensions, even in the axis with optical power, cannot be independently controlled. Furthermore, there is no focusing in the axis perpendicular to the lens which, for efficient coupling, constrains the dimensions of the optical fiber to equal or exceed that of the laser diode slow-axis emission width.
In general it is desirable to have optical power in both the fast-axis and slow-axis for most efficient coupling. Such systems have been demonstrated using a crossed pair of cylindrical lenses or a single microoptic fabricated with a different surface figure in each axis. In the prior art, for both these systems, the lens or lenses are positioned in a linear fashion between the laser diode and optical fiber.
Both of these systems have drawbacks. While the combination of crossed cylindrical lenses is efficient because it has optical power in both axes, it is very difficult to align. Moveover, light from the laser diode must pass through five optical surfaces before it is launched inside the fiber. For efficient coupling these surfaces would need to be anti-reflection coated, thus increasing the system cost. Regarding the single microoptic with different optical powers in orthogonal axes, it is not readily manufacturable with standard optical fabrication techniques. As a result, such an optical element is expensive relative to cylindrical lenses fabricated from drawing glass preforms. The single microlens also does not allow independent adjustment of beam lateral dimension and beam divergence.
An alternate coupling arrangement, used typically for coupling the output beam of a single diode-laser into an optical fiber, is a transverse coupling method. In this arrangement, the end of a fiber into which the output-beam is to be coupled is polished at an angle of about forty-five degrees to the longitudinal axis of the fiber. The fiber is oriented with its longitudinal axis perpendicular to the fast axis of the diode-laser, and close to the end-face of the diode-laser, such that the output-beam passes transversely through the fiber and is reflected internally from the angle-polished surface thereof along the fiber. This arrangement allows the cylindrical outer surface of the fiber itself to act as its own lens for focusing the fast-axis radiation of the output beam.
While this system is appealing in its simplicity it suffers from low collection efficiency. The refractive index and surface figure of a typical optical fiber is not optimal for focusing the fast-axis radiation. In the slow-axis no optical power is available to focus the beam, which constrains the fiber dimensions for efficient coupling. Further, it is impractical to coat the optical fiber with an anti-reflection coating because it is difficult to coat just the side surface of the fiber at its input end.
There is a need for a coupling arrangement which provides optical power in both axes of the diode-laser. Furthermore, there is a need to match the lateral beam dimensions in the fast-axis and slow-axis at the point where the beam in launched into the optical fiber. The coupling arrangement needs to provide these attributes with a minimum number of inexpensively fabricated optical components which have a minimum number of transmissive optical surfaces. These components need to be convenient to align, and have reasonable mechanical tolerances for optimum coupling efficiency.